The invention concerns a light-metering system in an attachment camera for observation instruments, especially microscopes, with an observation and metering beam path derived from the beam path of the observation instrument by means of an optical structural group, a diaphragm arranged in the image plane of the path of the metering beam and a photoelectric receiver.
In order to adapt the diaphragm to a section of the image of the image field of the observation instrument to be evaluated by optical-electrical means, it has already been proposed to design the diaphragm in a fully reflecting form and thus to bring an image of the object to be photometered into the path of the observation beam.
The disadvantage of such a device lies in the fact that it is possible to work with only a given diaphragm magnitude. The continuous adaption of the diaphragm to the object and the visualization of the magnitude of the diaphragm and its location in the image of the object is not possible.
It has also been proposed to arrange an iris diaphragm in front of the exposure meter of a microscope in order to equilibrate the light flux. The disadvantage here again is that the iris diaphragm cannot be seen in the path of the observation beam of the instrument.
It has further been proposed to provide graticules in the path of the observation beam, corresponding to the image section of the image registering on a photographic device.
However, such devices require settings of excessive accuracy. In addition, the subjective impression as compared with a genuine diaphragm reproduction, is poor.
Finally, devices are known in which a variable diaphragm is illuminated by a source of light from behind and in which both the beam reflected by the diaphragm and the beam coming from the object of the observation instrument are guided by a beam splitter into the observation portion of the device directly or by way of a full mirror.
The disadvantage of these devices consists of the fact that an additional source of light is used to illuminate the diaphragm, which light source also interferes with the light receiver used in the photoelectric evaluation (dark current behavior).